In general, crystalline forms of drugs are preferred over amorphous forms of drugs, in part, because of their superior stability. For example, in many situations, an amorphous drug converts to a crystalline drug form upon storage. Because amorphous and crystalline forms of a drug typically have differing physical/chemical properties, potencies and/or bioavailabilities, such interconversion is undesirable for safety reasons in pharmaceutical administration. A key characteristic of any crystalline drug substance is the polymorphic behavior of such a material. Polymorphs are crystals of the same molecule which have different physical properties because the crystal lattice contains a different arrangement of molecules. The different physical properties exhibited by polymorphs affect important pharmaceutical parameters such as storage, stability, compressibility, density (important in formulation and product manufacturing) and dissolution rates (important in determining bioavailability). Stability differences may result from changes in chemical reactivity (e.g., differential hydrolysis or oxidation, such that a dosage form discolors more rapidly when comprised of one polymorph than when comprised of another polymorph), mechanical changes (e.g., tablets crumble on storage as a kinetically favored crystalline form converts to thermodynamically more stable crystalline form) or both (e.g., tablets of one polymorph are more susceptible to breakdown at high humidity). Solubility differences between polymorphs may, in extreme situations, result in transitions to crystalline forms that lack potency or are toxic. In addition, the physical properties of the crystalline form may be important in pharmaceutical processing. For example, a particular crystalline form may form solvates more readily or may be more difficult to filter and wash free of impurities than other forms (i.e., particle shape and size distribution might be different between one crystalline form relative to other forms).
Agencies such as the United States Food and Drug Administration closely regulate the polymorphic content of the active component of a drug in solid dosage forms. In general, the regulatory agency requires batch-by-batch monitoring for polymorphic drugs if anything other than the pure, thermodynamically preferred polymorph is marketed. Accordingly, medical and commercial reasons favor synthesizing and marketing solid drugs as the thermodynamically stable polymorph, substantially free of kinetically favored polymorphs.
1-{[(α-Isobutanoyloxyethoxy)carbonyl]aminomethyl}-1-cyclohexane acetic acid, (1), a prodrug of the GABA analog gabapentin (2), has high bioavailability as gabapentin when dosed either orally or directly into the colon of a mammal (Gallop et al., International Publication No. WO 02/100347). The high bioavailability makes compound (I) a valuable component of oral dosage forms (including sustained-release dosage forms) useful for treating or preventing epilepsy, pain (especially, neuropathic pain and muscular and skeletal pain), depression, anxiety, psychosis, faintness attacks, hypokinesia, cranial disorders, neurodegenerative disorders, panic, inflammatory disease (i.e., arthritis), insomnia, gastrointestinal disorders, hot flashes, restless legs syndrome, urinary incontinence or ethanol withdrawal syndrome.

Compound (1), prepared as described in Gallop et al., International Publication No. WO 02/100347, is isolated as a glassy solid after lyophilization from aqueous acetonitrile. The material obtained by this process is partially or wholly amorphous and certain alkali metal salt forms are hygroscopic. However, amorphous solids and particularly hygroscopic solids are difficult to handle under pharmaceutical processing conditions because of low bulk densities and unsatisfactory flow properties. Moreover, handling of hygroscopic solids requires special techniques and equipment to obtain, for example, reproducible amounts of active compound or solid formulation stability. Furthermore, drugs that are hygroscopic must be packaged in special containers that have water vapor barriers thus substantially increasing the cost of such products.
Accordingly, a need exists for crystalline forms of 1-{[(α-isobutanoyloxyethoxy)carbonyl]aminomethyl}-1-cyclohexane acetic acid with superior physicochemical properties that may be used advantageously in pharmaceutical processing and pharmaceutical compositions.